The present invention relates to ionization cells of the type generally described in U.S. Pat. No. 3,835,328. Such cells can be used to detect the presence of very small concentrations of a select group of chemical vapors or gases in the air or in other vapor or gas backgrounds. Described in U.S. Pat. No. 3,835,328, is an ionization detector in which a gas sample flows past a source of ionization radiation and through a recombination region to an ion collection region, where a current is produced as a function of the ion concentration in the gas sample. By providing a sufficiently long path and an exposure to adequate surface, the recombination of ions is enhanced and controlled in such a way that ion concentration remaining in the gas sample when it reaches the collection region is a function of trace gases or vapors in the sample.
Deficiencies in the prior art design of ionization cells are that a variation in the rate of gas flow and/or a variation in the intensity of the ionizing radiation will result in a corresponding change in the concentration of ions reaching the collection region of the cell. To assure that the current monitored at the collector screen mounted in the collection region of the cell is a function of trace gases or vapors only, it is required that a very good constant rate pump be used to maintain a constant flow rate, or alternatively, that compensation be provided to correct for errors caused by changes in flow rate. In addition, compensation should be provided for a decrease in the intensity of the ionizing radiation due to natural radioactive decay and other factors, such as contaminating films. In the prior art, compensation flow rate changes are provided by utilizing a separate means for detecting flow changes, such as by using a second ionization cell sensitive only to flow changes. No compensation was provided in the prior art for source degradation.